The present invention relates to an apparatus for abrading a workpiece and while not limited thereto, the apparatus is particularly useful for lapping and polishing samples of semiconductive material to determine spreading resistance, resistivity and impurity concentration in the semiconductive sample.
In Mazur U.S. Pat. No. 3,628,137, there is described a technique for determining the foregoing parameters in a semiconductive sample by forming a surface at an angle with respect to the top of the sample in order that spreading resistance, for example, can be determined throughout the thickness of the sample. Mazur U.S. Pat. Nos. 3,277,610 and 3,426,484 disclose an apparatus and method for abrading workpieces in which the workpieces are freely movable on a polishing or lapping tray containing loose abrasive material. Eccentric motion is imparted to the tray by a rotary drive so that regions of the tray on which the workpieces are disposed undergo acceleration, thus moving the workpieces by inertial forces over the tray surface. The spacing of the workpieces is such that, in relation to the frictional forces on the workpieces exerted by the tray surface, as the workpieces are moved by inertial forces they contact each other so that the workpieces are randomly displaced over a substantial area during their movement. A resilient member is disposed along the outer periphery of the tray and suspended a slight distance above the tray by springs to facilitate continuous inertial movement. Rubber bands are stretched across the tray between opposing walls to subdivide the tray into compartments for retaining workpieces in the compartments. Mazur U.S. Pat. No. 3,978,622 discloses an abrading apparatus for workpieces wherein the workpiece is disposed on a lapping and polishing tray carried on a generally horizontal support plate. The plate is caused to oscillate or eccentrically rotate by a single, centrally-located eccentric arm. The plate is restrained against rotary movement about the eccentric arm by thrust bearings situated at the four corners of the tray and between the tray and a support housing.
The shuffling, random-type accelerations of the fixture and workpiece supported thereby along the tray to effect lapping and/or polishing may interrupt the application of uniform pressure between the surface of the sample of semiconductive material and the lapping or polishing surface. The fixture which has a substantial mass and extends several inches above the surface of the lapping or polishing surface of the plate can rock about its center of mass which is located above the lapping or polishing surface as the fixture is continuously accelerated by movement of the plate. The lapping or polishing operation on the sample of semiconductive material is carried out to produce a planar, beveled surface which, when measuring the spreading resistance, for example, forms an obtuse angle with an original planar surface of the specimen. Defective beveling of a sample of semiconductive material can take several forms. In one form, instead of a sharp intersection between the beveled surface and the original planar surface, a rounded bevel edge sometimes occurs between the two surfaces. The existence of a rounded transition zone makes it impossible to determine the beginning of the beveled surface. Also, in this transition zone, the depth of the specimen cross section which is being probed is not a linear function of stepped increments along the beveled surface.
Another form of defective beveling is a curved intersection between the beveled surface and the original planar surface with the curve lying in the plane of the original surface. This "bevel-edge arcing", as it is sometimes referred to in the art, is produced instead of a straight-line intersection between the original and beveled surfaces. When a curved intersection is produced between the beveled surface and the original surface, there is an indication that one or both of the surfaces are non-planar. When this defective beveling occurs, any means for determining the beveled angle which samples a finite width of the specimen along the arc will not provide a unique angle value. Shallower beveled angles are produced on thin specimens. The smaller angle means that there is a smaller included obtuse angle between the beveled surface which is produced on the specimen and the original planar surface.
Typical specimens have an overall thickness of 10-20 mils but, in many cases, the active layer of interest in the spreading resistance measurements can be as thin as a fraction of a micron. Defective beveling is more prevalent with specimens in which the layer of interest is so thin. The apparatus of the present invention eliminates the disadvantages of a motor-driven eccentric to impart rotary motion to a polishing plate and provides that the polishing surface is stationary to avoid imparting a vertical force component on a workpiece in a manner to eliminate tilting on the lapping or polishing plate so that the sample is always pressed by a uniform force against the plate. The apparatus is designed to assure non-defective beveling which is more difficult to achieve when beveling thinner specimens which must have a smaller beveled angle.